Two drive schemes commonly used with ferroelectric LCDs are the JOERS/Alvey scheme and the Malvern schemes. As described by PWH Surguy et al in Ferroelectrics 122, 63, 1991, the JOERS/Alvey drive scheme is for use with an LCD having a plurality of rows and columns of electrodes. A two time slot strobe pulse is applied to the rows and a data pulse is applied to the columns. One of the time slots of the strobe pulse is at zero, the other time slot having an amplitude Vs. The strobe pulse is scanned down the plurality of row electrodes.
The data pulse has an amplitude Vd and the polarity thereof may be changed between each slot.
At each pixel of the LCD, the effective applied electric field is the combination of the strobe pulse and the data pulse. In the time slot wherein the strobe pulse is zero, the magnitude of the effective electric field will be equal to Vd. However, in the other slot, the strobe and data pulses combine and depending upon their polarity, the resultant may have a magnitude greater or less than either of the strobe and data pulses. If the magnitude falls within a predetermined range, switching of the pixel occurs.
The Malvern schemes are similar to the JOERS/Alvey scheme, but instead of the strobe pulse being at zero for one time slot and at Vs for the other slot, the strobe pulse is at zero for one time slot and at Vs for several time slots. In order to distinguish between the different Malvern schemes; the schemes are identified by the number of slots over which the strobe pulse is at Vs, for example Malvern-2 denotes the scheme in which the strobe pulse is zero for one slot and at Vs for two slots. The Malvern schemes are described in Liquid Crystals 13, 597, 1993.
When used to control a ferroelectric LCD capable of displaying a plurality of grey scales, it is desirable to be able to apply a range of electric fields to the LCD. However, the above described drive schemes are intended for black and white operation.
GB-A-2 178 582 relates to a liquid crystal apparatus and driving method for addressing continuous or analogue grey levels.